The present invention relates to a device and to a method for optical filtering.
Various methods are known for improving the capacity of existing optical networks. One method is to use so-called wavelength multiplexing technology (WDM) to improve the extent to which an optical fibre in the optical network can utilise available bandwidths. The wavelength can also be used as an information address in an optical network, in other words the information can be multiplexed on a number of channels which can then be processed individually in the network. This can cause different channels to be subjected to losses of different magnitudes, among other things because the different channels are attenuated to different extents in the filter and switching structures, pass through the network along paths of mutually different lengths, or are amplified to different extents in optical amplifiers. This imbalance can impair the quality of the transmitted information, due to the fact that a channel that has a low power level is easily disturbed by a channel that has a high power level, this phenomenon normally being referred to as crosstalk.
Consequently, it is desirable to incorporate in an optical network tuneable filters which will enable undesirable channels to be suppressed while amplifying desired channels.
Devices constructed in accordance with the present standpoint of techniques for tuneable filtering of optical channels are generally encumbered with one or more of the following defects:
Relatively high losses with respect to desired channels and poor suppression of remaining channels.
Other defects include reflections in the device which impair performance and cause disturbances in the transmission system as a whole.
Another drawback is that wavelength channels are filtered only over a narrow wavelength band.
Another drawback is that these known devices have an over-sharp filter profile (not system-friendly).
An object of the present invention is to overcome the aforesaid problems and defects, at least partially.
This object is achieved in accordance with a first aspect of the invention by means of a device and a method for selective optical wavelength filtering. A filter includes a coupler or a circulator, a multiplexer/demultiplexer and a plurality of waveguides. Each of the plurality of waveguides includes a reflection section and a variable optical attenuator wherein the variable optical attenuator is disposed between the reflection section and the multiplexer/demultiplexer. One of the ports on the circulator or on the coupler is connected to a first side of the multiplexer/demultiplexer.
One advantage afforded by the present invention is that dispersion compensation can be achieved for each channel when the period in the grating structures is varied.
Another advantage afforded by the invention is that undesirable channels can be strongly suppressed.
One preferred embodiment affords the additional advantage of enabling desired channels to be amplified to great extent.